During state-of-the-art semiconductor processing of semiconductor devices, many devices are made in a single substrate. These devices are connected to each other by means of conductive lines. However, since these conductive lines can introduce unwanted electric signals in the semiconductor substrate during operation of the devices, the devices must be separated from each other by some means of isolation. The usual means of isolation is to etch trenches between the devices that can be filled with an isolation material.
In order to maintain the critical dimension control of the active area, the sidewalls of the nitride layer must be straight, requiring an anisotropic etch; the etch chemistry must be such that unwanted materials do not deposit on the sidewalls or the bottom of the trench (such as polymeric compositions) and the bottom of the trench must have rounded bottoms and no microtrenching.
Conventional etchants for silicon nitride include fluorocarbons, e.g., freons, and hydrohalides, such as HBr or HCl. The use of freons is being discouraged for environmental reasons, and the use of hydrogen halides causes formation of polymeric residues, and deposits the residues on the sidewalls and bottom of the etched trench as well as on the chamber walls. Thus a good etchant composition for etching silicon nitride for shallow trench formation must produce straight walled openings, must not form deposits on the sidewalls or bottom of the trench, and must be anisotropic throughout. These stringent requirements for an etchant are difficult to achieve and have continued to be sought.